System and method for fabricating high voltage power mosfet

ABSTRACT

A high voltage power MOSFET includes a semiconductor substrate doped by a first conducting type, a source doped by a second conducting type and over the semiconductor substrate, and a drain region doped by the second conducting type and on the semiconductor substrate. One or more drain layers doped by the second conducting type and on the semiconductor substrate span between the body region and the drain region. An insulating layer is formed on at least a portion of the body region and over the one or more drain layers. A voltage regulating layer on the insulating layer can produce voltage distributions in the one or more drain layers to deplete charge carriers to increase blockage voltage in an off state, and to accumulate charge carriers in an on state to reduce on-state resistance.

BACKGROUND OF THE INVENTION

This application relates to semiconductor devices, and moreparticularly, to high voltage power metal-oxide-semiconductorfield-effect transistors (MOSFET).

One of the most important components in a high voltage semiconductorpower MOSFET is the lightly doped drain (LDD) region (also known as“drift” region). While LDD helps a device supporting high appliedvoltage during the voltage blocking “off” state, it also heavily limitsthe current conductivity during “on” state then current is conducting.In a conventional high voltage power MOSFET, due to the LDD resistance,the total on-resistance (R_(on)) rapidly increases with the desiredreverse breakdown voltage (V_(b)). The relationship can be roughlyexpressed as a function of R_(on)˜V_(b) ³. For example, in a 750Vconventional vertical MOSFET, the LDD contributes almost 99% of thedevice's total on-resistance.

There have been a few methods developed over the years to reduce theon-resistance. They help continually driving down device unit areaon-resistance, or specific on-resistance. Most methods attempt toincrease LDD doping density for a given reverse breakdown voltage. Oneof the examples is reduced surface field (RESURF), which utilizespartial charge compensation or conductor field plates to reduce peakelectrical field, it allows a higher total LDD doping profile to be used(J. A. Appels and H. M. J. Vaes, HV Thin Layer Devices (RESURF Devices),Proc. Intl. Electron Devices Meeting, pp. 238-241, 1979). Another methodintroduces the so-called charge balance (CB) to help deplete LDD beforeits breakdown (Coe, U.S. Pat. No. 4,754,310A; Chen, U.S. Pat. No.5,216,275; and Tihanyi, U.S. Pat. No. 5,438,215). Charge balance methodhas been mainly used in vertical MOSFET and over the years it hasgradually reduced specific on-resistance. The RESURF and field controlmethods are more widely used in lateral MOSFET that allow easyintegration with lower voltage devices. Over the years, they havegradually evolved into so-called multiple RESURF by combining chargebalance effects. Charge balance and multiple RESURF are currentlyproducing the best specific on-resistance R_(s) in integrated MOSFET.They rely on columns of P and N type materials that deplete each otherwhen biased into off-state. In general, doping density, and thusconductivity, can be increased when column width is reduced. But theyare relatively difficult to mass produce, due to the difficulty in bothcolumn width and doping density control, especially the latter thatrequires P and N types be closely matched. Due to these limitations,multiple RESURF method finds only limited use in production.

In pursuing the same goal of more effectively depleting LDD, other fieldmanipulating techniques have been proposed. But in general, they aremuch less successful. For example, in Kocon, U.S. Pat. No. 6,717,203,bias electrodes disposed adjacent the drift (LDD) region to alter theelectric field in the drift region to increase reverse breakdownvoltage. This approach is relatively complex, and the extra controllingterminals make it difficult to be implemented in applications. InDarwish, U.S. Pat. No. 8,592,906, capacitively coupled floatingelectrodes in LDD provide a similar means to deplete the LDD withincreasing applied voltage, but it has the disadvantage of not beingstable enough due to the presence of floating components.

There is therefore a need for an improved advanced on art to have animproved semiconductor high voltage power MOSFET that is simple toimplement and provides control stability.

SUMMARY OF THE INVENTION

The present invention pertains to high voltage semiconductor powerMOSFET devices. In the present disclosure, reverse breakdown voltage forsuch devices can be increased by embedding electrical field in the LDDregion while maintaining relatively high conductivity. A controlmechanism of embedding structures also allows LDD to be driven intoaccumulation in on-state, which results in significant on-resistancereduction. At off-state, a current conducting path above or surroundingLDD region, between the MOSFET drain and source, provides a stableembedded electrical field depleting charge carriers from the LDD region.The conducting path is separated from the LDD region by a thin layer ofinsulating dielectric material. During the voltage blocking period, oroff-state, a small current flows through the path, being resistors orjunction devices, provides a stable potential distribution and embeddedelectrical field depleting the LDD region of charge carriers. Thedepletion in turn enhances the blocking voltage. The path can beadjusted to produce a desired off-state electrical field distributionwithin LDD and optimize blocking voltage. The depletion effect allowsthe use of an extra optional thin layer of conductive material (extraLDD doping) near the surface to enhance device conductivity in on-statemode. The embodiments can also be manipulated to further significantlyimprove conductivity by biasing LDD into accumulation during on-state.As accumulation normally increases carrier density a few orders ofmagnitude, the on-resistance is significantly reduced.

The current conducting path in present invention can be a resistivematerial, or a series planar junction diodes connected in series,controlled by the MOSFET source and drain. Some disclosed devices havethe path controlled by MOSFET gate and can be biased in accumulation inconduction mode, which results in significant on-resistance reduction.

One advantage of present invention is that the disclosed devices can beextended to three-dimensional embodiments. In addition to the abovementioned one-dimensional depletion effect, a three-dimensionalstructure also depleted laterally by the two side conducting paths. Forthe conducting paths using diodes, the depletion during off-state andaccumulation during on-state are enhanced when the distances betweenadjacent two diodes are reduced. It is thus can be scaled withtechnology progress. With progress in semiconductor process technology,the semiconductor dimensions could be reduced and conductivityincreased.

In one general aspect, the present invention relates to a high voltagepower MOSFET that includes a semiconductor substrate doped by a firstconducting type; a source doped by a second conducting type and over thesemiconductor substrate; a drain region doped by the second conductingtype and on the semiconductor substrate; one or more drain layers dopedby the second conducting type and on the semiconductor substrate,wherein the one or more drain layers span between the body region andthe drain region, wherein doping levels in the source and in the drainregion are at least an order of magnitude higher than doping levels inthe body region and the one or more drain layers; a body region doped bythe first conducting type and configured to separate the source from theone or more drain layers and the drain region; an insulating layerformed on at least a portion of the body region and over the one or moredrain layers; a voltage regulating layer on the insulating layer andconfigured to produce voltage distributions in the one or more drainlayers to deplete charge carriers to increase blockage voltage in an offstate, and to accumulate charge carriers in an on state to reduceon-state resistance. The voltage regulating layer can include aresistive layer or a layer of serially connected P-N junctions; and agate on the insulating layer and at least in part over the source.

Implementations of the system may include one or more of the following.The insulating layer can be on and covers the section of the body regionthat is sandwiched between the source and the one or more drain layers.Doping levels in the source and in the drain region can be at least twoorders of magnitude higher than doping levels in the body region and theone or more drain layers. Doping levels in the source and in the drainregion can be at least an order of magnitude higher than doping levelsin the semiconductor substrate. The first voltage regulating layer caninclude a resistive layer, wherein the resistive layer can include afirst end electrically connected with the source and a second endelectrically connected with the drain region. The first voltageregulating layer can include a resistive layer, wherein the resistivelayer can include a first end electrically connected with the gate and asecond end electrically connected with the drain region. The firstvoltage regulating layer can include a resistive layer, wherein theresistive layer includes a first end electrically connected with thegate and a second end, wherein the high voltage power MOSFET can furtherinclude a P-N junction diode connected between the second side of theresistive layer and the drain region. The first voltage regulating layercan include a layer of serially connected P-N junctions. The layer ofserially connected P-N junctions can include multiple pairs of P-Njunctions connected back-to-back. The layer of serially connected P-Njunctions can include a first end electrically connected with the sourceand a second end electrically connected with the drain region. The layerof serially connected P-N junctions can include a first end electricallyconnected with the gate and a second end electrically connected with thedrain region. The layer of serially connected P-N junctions can includemultiple serially connected uni-directional P-N junctions. The layer ofserially connected P-N junctions can include a first end electricallyconnected with the gate and a second end, wherein the high voltage powerMOSFET can further include: a P-N junction diode connected between thesecond end of the layer of serially connected P-N junctions and thedrain region. The voltage regulating layer can include a first layer ofserially connected P-N junctions, wherein the high voltage power MOSFETcan further include: one or more second layers of serially connected P-Njunctions connected in parallel to the first layer of serially connectedP-N junctions, wherein the first layer of serially connected P-Njunctions and the one or more second layers of serially connected P-Njunctions can produce voltage distributions in the one or more drainlayers to deplete charge carriers to increase blockage voltage in an offstate, and to accumulate charge carriers in an on state to reduceon-state resistance. The voltage regulating layer can include a layer ofat least two serially connected P-N junctions that are connected inparallel with each other, wherein the at least two serially connectedP-N junctions can produce voltage distributions in the one or more drainlayers to deplete charge carriers to increase blockage voltage in an offstate, and to accumulate charge carriers in an on state to reduceon-state resistance. Each of the at least two serially connected P-Njunctions can include multiple serially connected uni-directional P-Njunctions. Each of the at least two serially connected P-N junctions caninclude multiple pairs of P-N junctions connected back-to-back. Thefirst conducting type can be P type, wherein the second conducting typeis N type, wherein the high voltage power MOSFET is a high voltageN-type power MOSFET. The first conducting type can be N type, whereinthe second conducting type is P type, wherein the high voltage powerMOSFET is a high voltage P-type power MOSFET.

These and other aspects, their implementations and other features aredescribed in details in the drawings, the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are circuit diagrams of high voltage power MOSFETs comprisinga resistive layer to provide voltage distributions to reduce on-stateresistance in accordance with some embodiments of the present invention.

FIGS. 4-5 are circuit diagrams of high voltage power MOSFETs comprisinga layer of serially back-to-back connected P-N junctions for reducingon-state resistance in accordance with some embodiments of the presentinvention.

FIG. 6 is a circuit diagram of a high voltage power MOSFET comprising alayer of serially connected uni-directional P-N junctions for reducingon-state resistance in accordance with some embodiments of the presentinvention.

FIG. 7 is a circuit diagram of a high voltage power MOSFET comprisingtwo layers of serially connected uni-directional P-N junctions toprovide voltage distributions to reduce on-state resistance inaccordance with some embodiments of the present invention.

FIG. 8 is a partial top view of two series of uni-directional P-Njunctions in parallel formed in a layer to provide voltage distributionsto reduce on-state resistance in a high voltage power MOSFET inaccordance with some embodiments of the present invention.

FIGS. 9 to 12 are exemplified equivalent circuits to perform similarfunctions to the two serially connected P-N sections in FIGS. 7 and 8 toprovide voltage distributions to reduce on-state resistance in a highvoltage power MOSFET in accordance with some embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone of ordinary skill in the art that the present invention may bepracticed without these specific details. In other instances, well knownmethods, procedures, components, and circuits have not been described indetail as not to unnecessarily obscure aspects of the present invention.

Referring to FIG. 1, a high voltage N-type power MOSFET 10 includes alightly P⁻ doped semiconductor substrate 100, and a heavily N⁺ dopedsource 130 and a heavily N⁺ doped drain region 170 in the lightly P⁻doped semiconductor substrate 100. An N-type lightly doped drain (LDD)layer 150 that spans between the N⁺ doped drain region 170 and a P-typebody region 140 adjacent to the heavily N⁺ doped source 130 insupporting applied high voltage. An optional thin N-type lightly dopeddrain (LDD) layer 160 can be formed on the LDD layer 150. The P-typebody region 140 isolates the N⁺ doped source 130 from the LDD layer150/160 and the N⁺ doped drain region 170.

In some embodiments, the doping levels in the N⁺ doped source 130 and inthe N⁺ doped drain region 170 are at least one order magnitude higherthan doping levels in the P-type body region 140 and the one or moreN-type doped drain layers 150/160. In some embodiments, the dopinglevels in the N⁺ doped source 130 and in the N⁺ doped drain region 170are at least two orders of magnitude higher than doping levels in theP-type body region 140 and the one or more N-type doped drain layers150/160.

The doping level for the LDD layer 150 can vary from material tomaterial, for example, ≦10¹⁷/cm³ in Silicon, but can be different inother materials. The doping level for the heavily doped N⁺ source 130and the N⁺ doped drain region 170 can be in a range >1×10¹⁹/cm³ inSilicon.

In the present disclosure, the higher doping regions such as the N⁺doped source 130 and the N⁺ doped drain region 170 are denoted as N⁺ ,which in silicon can have doping range of ˜10¹⁹-10²⁰/cm³ in silicon. TheP-type body region 140 can have a doping range of ˜10¹⁷-10¹⁸/cm³ insilicon. The N⁻ lightly doped drain (LDD) layers 150, 160 has a dopinglevel range of ˜10¹⁵-10¹⁸/cm³ in silicon. The lightly P⁻ dopedsemiconductor substrate 100 has a doping level range of ˜10¹⁴-10¹⁷/cm³in silicon.

In some embodiments, the lightly P⁻ doped semiconductor substrate 100have doping level similar to or lower than those in the P-type bodyregion 140 and the one or more N-type doped drain layers 150/160. Inother words, the doping levels in the N⁺ doped source and in the N⁺doped drain are at least two orders of magnitude higher than dopinglevels in the P-doped semiconductor substrate.

An insulating layer 120 is formed on at least a portion of the P-typebody region 140 and on the LDD layer 160 and on or over the LDD layer150. Specifically, the insulating layer 120 is on and covers the sectionof the P-type body region 140 that is sandwiched between the N⁺ dopedsource 130 and the one or more N-type doped drain layers 150/160.

An electrode gate 110 on the insulating layer 120, over the P-type body140 and at least in part over the source 130 provides a mean to controlthe conductivity of the MOSFET. When a positive voltage is applied tothe gate 110, the top layer of P-type body region 140 is inverted toN-type, generating a conducting channel between the source 130 and theLDD layer 150 and the N⁺ doped drain region 170 and the device isoperated in on-state. Setting gate 110 voltage to zero turns it intooff-state, with which a high voltage can be applied.

The presently disclosed high voltage N-type power MOSFETs includes avoltage regulating layer that can produce voltage distributions in theone or more N-type lightly doped drain layers to deplete charge carriersto increase blockage voltage in an off state, and to accumulate chargecarriers in an on state to reduce on-state resistance. The voltageregulating layer can include a resistive layer or one or more layers ofserially connected P-N junctions.

In some embodiments, a resistive layer 200 made of highly resistivematerial is formed on the LDD layer 150 and over the insulating layer120. Conductors 190, 191 at the two ends of the resistive layer 200 of ahighly resistive material respectively connects to the N⁺ doped drainregion 170 and the source 130. The equivalent circuit for the resistivelayer 200 is shown in the dashed circle on the top portion of FIG. 1,which includes a distributive resistor 200′ having its two endsconnecting to the conductors 190, 191. At the off-state, a small currentflowing through the distributive resistor 200′ generates a graduallydecreasing voltage above the LDD layer 150, which forces an evenlydistributed electrical field on LDD layer 150 that depletes the chargecarriers in LDD layer 150, which prevents a premature breakdown. Herethe resistive layer 200 is to be maintained at a high resistive valueand thin thickness, so the current is below the limits required byspecific applications. The depletion allows the higher doped LDD layer150 to support an applied voltage higher than it normally can. Theoptional conducting layer LDD layer 160 below the resistive layer 200and the insulating layer 120 can be easily depleted thoroughly andshould not affect the breakdown voltage. The higher doping levels in theLDD layer 150 and extra doping from the LDD layer 160 result in a higherconductivity at the on-state. To effectively deplete LDD layers 150 and160, the voltage distribution is very critical. The resistive layer 200adequately provides a desired voltage distribution.

It should be noted that although high voltage N-type power MOSFETs havebeen described above and below, the present invention is compatible withP-type high voltage power MOSFETs, which can be implemented by reversingthe doping polarities of the semiconductor substrate, the body region,the source, the drain region and the drain layers, etc. from thedisclosed examples. Various regions and layers in the N-type powerMOSFETs are doped by two conducting types: N-type and P-type. In theP-type power MOSFETs, the conducting types in the corresponding regionsand layers are switched.

In some embodiments, referring to FIG. 2, a high voltage N-type powerMOSFET 20 includes some components similar to those in the high voltageN-type power MOSFET 10 in FIG. 1 except for the resistive layer 200 iselectrically connected to the N⁺ doped drain region 170 and the gate110. The high voltage N-type power MOSFET 20 operates similar to thehigh voltage N-type power MOSFET 10 at off-state, but quite differentlyin on-state. At the off-state, small current flows through the resistivelayer 200 and forces an evenly distributed electric field on the LDDlayer 150 (and 160) that depletes the charge carrier, which in turnprevents a premature breakdown. In the on-state, a gate bias is appliedto both the gate 110 and the resistive layer 200. This applied gate biasvoltage gradually decreases in the resistive layer 200 from the gate 110to the N⁺ doped drain region 170. Because of this gradually decreasingvoltage distribution is immediately above isolation layer 120, the LDDlayers 150 and 160 are biased into charge-carrier accumulation mode,with sections near the gate 110 heavily accumulated and sections nearthe N⁺ doped drain region 170 lightly accumulated. With charge-carrieraccumulation within LDD layers 150 and 160, the carrier density issignificantly increased, especially near the gate 110 where carrierdensity can be orders of magnitude higher. As a result, conductivity issignificantly increased and the device on-state resistance reduced. Theequivalent circuit of resistive material for the resistive layer 200 isshown in dashed circle in the top portion of the FIG. 2.

Similarly, referring to FIG. 3, a high voltage N-type power MOSFET 30 issimilar to the high voltage N-type power MOSFET 20 (FIG. 2). A P-Njunction diode 291 with reverse breakdown voltage higher than the normalgate bias is inserted between the resistive layer 200 and the N⁺ dopeddrain region 170. The high voltage N-type power MOSFETs 20 and 30operate similarly at the off-state. During the on-state, the wholeresistive layer 200 attains the same voltage as the gate 110 because itis isolated by the diode 291 from the N⁺ doped drain region 170. Withthe full gate voltage applied to the resistive layer 200, the LDD layers150 and 160 are biased into heavy accumulation, in which carrier densityis normally increased by several orders of magnitude and on-resistanceis significantly reduced. The equivalent circuit, which is an additionto a conventional high voltage power MOSFET, is shown in dashed circlein the top portion of FIG. 3.

In some embodiments, referring to FIG. 4, a high voltage N-type powerMOSFET 40 includes some components similar to those in the high voltageN-type power MOSFET 10 (FIG. 1) except for that the resistive layer 200on the insulating layer 120 is replaced by a series of planar P-Njunctions 300 comprising a series of P sections 310 and N sections 320.The equivalent circuit for planar P-N junctions 300 is shown on the topin the dashed circle, in which a series of diodes 300′ representing P-Njunctions have their two ends connected to the source 130 and the N⁺doped drain region 170.

The series of planar P-N junctions 300 is set to have a total breakdownvoltage equal to or slightly lower than that of underlying device. Withcurrent flowing through P-N junctions between drain and source prior tobreakdown, a gradually step-down potential and a relatively evenlydistributed electrical field is obtained, which in turn depletes LDDregion at off-state and support high applied voltage.

The P-N junctions 300 allow much lower off-state current, which isdesirable in most applications. On the other hand, its induced fielddistribution is less uniform than that in FIG. 1. However, so long asthe P sections 310 and the N sections 320 are small enough and theinsulating layer 120 properly chosen, the voltage distribution canprovide an adequate field that effectively depletes the LDD layers 150and 160 and supports a high voltage. In some embodiments, all junctiondiodes 300′ are substantially identical. The total junction breakdown isset to be lower or equal to the designed MOSFET breakdown. According tothe equivalent circuit, too high a breakdown voltage for the diodes 300′could result in a higher than necessary voltage accumulation near the N⁺doped drain region 170, which may generate an undesired field spike. Theplanar junction diode 300 can be formed by crystalline ornon-crystalline materials. For non-crystalline junction diodes likeamorphous or polycrystalline silicon or other semiconductor materialsdiodes, relatively high leakage current provides a current path similarto the resistive layer 200 in the high voltage N-type power MOSFET 10(FIG. 1). In these cases, the above mentioned diode breakdown voltagelimitation can be relaxed.

In some embodiments, referring to FIG. 5, a high voltage N-type powerMOSFET 50 includes some components similar to those in the high voltageN-type power MOSFET 20 (FIG. 2) except for that the resistive layer 200on the insulating layer 120 is replaced by a series of planar P-Njunctions 300 comprising a series of P sections 310 and N sections 320.The series of planar P-N junctions 300 is electrically connected to thegate 110 at one end and the N⁺ doped drain region 170 at the other end.

The planar P-N junction series is set to have a total breakdown voltageequal or slightly lower than that of underlying device. At off-state,small current flows through the series of planar P-N junctions 300 priorto breakdown, which produces a gradually step-down potential and arelatively evenly distributed electrical field in the LDD layer 150 (and160) and depletes the charge carriers therein, which in turn prevents apremature breakdown. During the on-state, the planar P-N junctions 300near the gate 110 attain some finite potential (i.e. higher-than-zerovoltage), which can bias part of the LDD layers 150/160 near the gate110 into accumulation and enhance conductivity. Shown on top (in dashedcircle) is the equivalent circuit of serial planar P-N junctions.

To make the on-state conductivity more effective, referring to FIG. 6, ahigh voltage N-type power MOSFET 60 includes a series of uni-directionalplanar P-N junctions 300 comprising a series of P sections 310 and Nsections 320. In the equivalent circuit above, a series of diodes 300′arranged in the same forward direction. A diode 340 representing a P-Njunction is reversely connected with the end of the series of diodes300′ and the N⁺ doped drain region 170. The diode 340′ is set to have areverse breakdown voltage higher than normal gate bias voltage. In thiscircuit configuration, assuming the diodes 300′ are ideal with zeroforward voltage, all diodes 300′ attain the same voltage as the gate 110during on-state. With a fully gate voltage applied over the insulatingdielectric layer 120, the LDD layers 150 and 160 are biased into heavycarrier accumulation, in which LDD carrier density is increased byseveral orders of magnitude and on-resistance is significantly reduced.The equivalent circuit for the layer of uni-directional P-N sections isshown on top in dashed circle.

In reality, all diodes have a finite forward voltage drop. At on-state,the voltage for a diode that is n diodes away from the gate 110 isV_(g)-nV_(df), where V_(g) is the applied gate voltage and V_(df) is theforward voltage drop for the diodes 300′. Those diodes 300′ far awayfrom the gate 110 receive very low or no bias. So this configuration islimited to cases of low count of diodes 300, which likely is limited torelatively lower voltage applications. For example, in silicon (Si), P-Njunction diode V_(df) is close to 0.7 V, for a typical gate bias voltageof 20V, n has to be less than 20 to ensure at least 6V on all diodes.

FIG. 7 shows another high voltage N-type power MOSFET 70 that includes aseries of uni-directional planar P-N junctions 300 comprising a seriesof P sections 310 and N sections 320, and in addition, a second layer ofdiodes 350 on the series of uni-directional planar P-N junctions 300.The diodes 350 are formed with different P sections 360 and N sections370 which have a reverse breakdown voltage higher than the summation ofthe reverse breakdown voltage of the respective diodes 300 under eachdiode 350. At one end, the series of uni-directional planar P-Njunctions 300 and the second layer of diodes 350 on top are bothconnected to the gate 110 via the conductor 190. The second layer ofdiodes 350 can be formed only on a portion of the series ofuni-directional planar P-N junctions 300 such that the other end of thesecond layer of diodes 350 is connected to a uni-directional planar P-Njunction 300 underneath via a conductor 191. The equivalent circuit forthe two layers of diodes is shown on top in dashed circle.

When operated in off-state, a current flows through diodes 300′ toprovide the necessary voltage and field distributions for depletingcharge carriers in the LDD layers 150 and 160. When operated inon-state, applied gate voltage V_(g) is fed through diode 350′ to diodes300′ that are far away from gate 110. As forward voltage V_(df) are thesame for the diodes 350′ and the diodes 300′, by by-passing theunderlying diodes 300′, n and nV_(df) are reduced, sufficiently highvoltages are applied to the diodes 300′ under the diodes 350′ far fromthe gate 110 and effective accumulation can be produced.

Referring to FIG. 7, two layers of planar diodes are used to form theequivalent circuit for producing the voltage distribution. Similar toconfigurations shown in FIGS. 9 to 12, other multiple-layerconfigurations can be compatible with the present disclosed power MOSFETdevices. These multiple-layer approaches will require additionalproduction steps and have extra cost associated with it. One of thesolutions is to rearrange them into a single layer.

While the high voltage N-type power MOSFET 70 include two layers ofuni-directional diodes for producing voltage distribution. The twoseries of uni-directional P-N sections can also be formed in a singlelayer in parallel to provide the necessary voltage distribution fordepleting charge carriers. Referring to FIG. 8, the uni-directional P-Nsections 300 and 350 are laid out side by side in a same layer on theinsulating layer 120 and the LDD layers 150 and 160. The electricalconnections and operations are similar to those multi-layer seriallyconnected P-N sections as shown in FIG. 7.

The additional cascaded diodes 350′ in FIGS. 7 and 8 produce a gate biasfeeding path to diodes 300′ for on-state operation. For devices of veryhigh reverse breakdown voltages, a large number of the diodes 300′ areneeded. The presently disclosure provides several circuit layouts whichcan reduce the number of diodes in the secondary series, as shown inFIGS. 9 to 12. The Equivalent circuits shown in FIGS. 9 to 12 arecompatible with multiple layers of serially connected P-N sections (inanalogy to FIG. 7), or multiple serially connected P-N sections inparallel in a single layer (in analogy to FIG. 8), over the insulatinglayer 120 and the LDD layers 150 and 160.

In FIG. 9, each node between diode 300′ is connected individually by adiode 365′ to the gate 110. Here, the reverse breakdown voltages ofupper diode 365′ can be the same or higher than the total breakdownvoltage of underlying diodes 300′. The diode 340′ is set to have areverse breakdown voltage higher than normal gate bias voltage. Thedesired voltage distribution in off-state is obtained through the bottomline of diodes 300′, with the upper diodes 365′ for gate bias aton-state only. At the on-state, all diodes 300′ receive almost the samevoltage as the gate 110 voltage and very effectively bias the LDD layers150/160 into charge carrier accumulation.

Similarly, referring to FIG. 10, a series of diodes 300′ are connectedback-to-back. Several upper diodes 366′ are connected in parallel withthe back-to-back connected diodes 300′. This is especially useful forproduction as connecting metal between diodes 300′ is omitted. The upperdiode 366′ can have a same or higher breakdown voltage than the totalbreakdown voltage of underlying diodes 300′. The diode 340′ is set tohave a reverse breakdown voltage higher than normal gate bias voltage.The desired voltage distribution in off-state is obtained through thebottom line of diodes 300′, with the upper diodes 366′ for gate bias aton-state only. At the on-state, all diodes 300′ receive almost the samevoltage as the gate 110 voltage and very effectively bias the LDD layers150/160 into charge carrier accumulation.

FIG. 11 is similar to FIG. 10, but with more than one upper diode 367′is cascaded for some diodes 300′ far from the gate 110. This circuitlayout allows the usage of diodes 367′ of lower reverse breakdownvoltages and helps reducing the diode types needed to build thestructure, which is important in designs and production as it simplifiesthe production process. Here, the total reverse breakdown voltages ofmultiple upper diode 367′ can be the same or higher than the totalbreakdown voltage of underlying diodes 300′. The diode 340′ is set tohave a reverse breakdown voltage higher than normal gate bias voltage.The desired voltage distribution in off-state is obtained through thebottom line of diodes 300′, with the upper diodes 367′ for gate bias aton-state only. At the on-state, all diodes 300′ receive sufficientlyhigh voltage and very effectively bias the LDD layers 150/160 intocharge carrier accumulation. This cascaded upper diode 367′ can also beapplied to the cases of FIG. 9 of unidirectional diodes 300′.

FIG. 12 is similar to FIG. 11, except for some upper diodes 368′ areconnected in series. Here, the total reverse breakdown voltages ofserial upper diode 368′ can be the same or higher than the totalbreakdown voltage of underlying diodes 300′. The diode 340′ is set tohave a reverse breakdown voltage higher than normal gate bias voltage.The desired voltage distribution in off-state is obtained through thebottom line of diodes 300′, with the upper diodes 368′ for gate bias aton-state only. At the on-state, all diodes 300′ receive sufficientlyhigh voltage and very effectively bias the LDD layers 150/160 intocharge carrier accumulation. This embodiment again allows fewer diodetypes to be used. This cascaded upper diode 368′ arrangement can also beapplied to the cases of FIG. 9 of unidirectional diodes 300.

Only a few examples and implementations are described. Otherimplementations, variations, modifications and enhancements to thedescribed examples and implementations may be made without deviatingfrom the spirit of the present invention. For example, the P-type highvoltage power MOSFET can be implemented by reversing the dopingpolarities of the semiconductor substrate, the body region, the source,the drain region and the drain layers from the examples disclosed above.The examples disclosed are also suitable for high voltage power MOSFETof various semiconductor materials. Moreover, vertical and lateral highvoltage power MOSFETs, with three dimensional voltage regulating layers,can be achieved by other means from the exemplified implementationsdescribed.

What is claimed is:
 1. A high voltage power MOSFET, comprising: asemiconductor substrate doped by a first conducting type; a source dopedby a second conducting type and over the semiconductor substrate; adrain region doped by the second conducting type and on thesemiconductor substrate; one or more drain layers doped by the secondconducting type and on the semiconductor substrate, wherein the one ormore drain layers span between the body region and the drain region; abody region doped by the first conducting type and configured toseparate the source from the one or more drain layers and the drainregion; an insulating layer formed on at least a portion of the bodyregion and over the one or more drain layers; a voltage regulating layeron the insulating layer, wherein the voltage regulating layer includes afirst end electrically connected with the source and a second endelectrically connected with the drain region; and a gate on theinsulating layer and at least in part over the source.
 2. The highvoltage power MOSFET of claim 1, wherein the insulating layer is formedon and covers the section of the body region that is sandwiched betweenthe source and the one or more drain layers.
 3. The high voltage powerMOSFET of claim 1, wherein the voltage regulating layer includes aresistive layer.
 4. The high voltage power MOSFET of claim 3, whereincross-sectionally the resistive layer includes a continuous portionbetween the first end and the second end.
 5. The high voltage powerMOSFET of claim 1, wherein the first voltage regulating layer includes alayer of serially connected P-N junctions.
 6. The high voltage powerMOSFET of claim 5, wherein the layer of serially connected P-N junctionsincludes multiple pairs of P-N junctions connected back-to-back.
 7. Thehigh voltage power MOSFET of claim 5, wherein the layer of seriallyconnected P-N junctions includes multiple serially connecteduni-directional P-N junctions.
 8. The high voltage power MOSFET of claim1, wherein the voltage regulating layer includes a first layer ofserially connected P-N junctions, the high voltage power MOSFET furthercomprising: one or more second layers of serially connected P-Njunctions connected in parallel to the first layer of serially connectedP-N junctions.
 9. The high voltage power MOSFET of claim 1, wherein thevoltage regulating layer includes at least two serially connected P-Njunctions that are connected in parallel with each other within thevoltage regulating layer.
 10. A high voltage power MOSFET, comprising: asemiconductor substrate doped by a first conducting type; a source dopedby a second conducting type and over the semiconductor substrate; adrain region doped by the second conducting type and on thesemiconductor substrate; one or more drain layers doped by the secondconducting type and on the semiconductor substrate, wherein the one ormore drain layers span between the body region and the drain region; abody region doped by the first conducting type and configured toseparate the source from the one or more drain layers and the drainregion; an insulating layer formed on at least a portion of the bodyregion and over the one or more drain layers; a voltage regulating layeron the insulating layer including a layer of serially connected P-Njunctions comprising multiple serially connected uni-directional P-Njunctions; and a gate on the insulating layer and at least in part overthe source.
 11. The high voltage power MOSFET of claim 10, wherein thevoltage regulating layer includes at least two serially connected P-Njunctions that are connected in parallel with each other within thevoltage regulating layer.
 12. The high voltage power MOSFET of claim 10,wherein the voltage regulating layer includes a first end electricallyconnected with the source and a second end electrically connected withthe drain region.
 13. The high voltage power MOSFET of claim 10, whereinthe layer of serially connected P-N junctions includes a first endelectrically connected with the gate and a second end electricallyconnected to the drain region.
 14. The high voltage power MOSFET ofclaim 10, wherein the voltage regulating layer is configured toaccumulate charge carriers in an on state to reduce on-state resistance.15. A high voltage power MOSFET, comprising: a semiconductor substratedoped by a first conducting type; a source doped by a second conductingtype and over the semiconductor substrate; a drain region doped by thesecond conducting type and on the semiconductor substrate; one or moredrain layers doped by the second conducting type and on thesemiconductor substrate, wherein the one or more drain layers spanbetween the body region and the drain region; a body region doped by thefirst conducting type and configured to separate the source from the oneor more drain layers and the drain region; an insulating layer formed onat least a portion of the body region and over the one or more drainlayers; a first voltage regulating layer on the insulating layer; asecond voltage regulating layer over the first voltage regulating layer;and a gate on the insulating layer and at least in part over the source,16. The high voltage power MOSFET of claim 15, wherein at least one ofthe first voltage regulating layer and the second voltage regulatinglayer includes a layer of serially connected P-N junctions.
 17. The highvoltage power MOSFET of claim 16, wherein the first voltage regulatinglayer includes a first layer of serially connected P-N junctions, andwherein the second voltage regulating layer includes a layer of seriallyconnected P-N junctions.
 18. The high voltage power MOSFET of claim 15,wherein at least one of the first voltage regulating layer and thesecond voltage regulating layer comprises multiple serially connecteduni-directional P-N junctions.
 19. The high voltage power MOSFET ofclaim 15, wherein at least one of the first voltage regulating layer andthe second voltage regulating layer comprises multiple pairs of P-Njunctions connected back-to-back.
 20. The high voltage power MOSFET ofclaim 15, wherein the first voltage regulating layer includes a firstend electrically connected with the gate and a second end electricallyconnected with the drain region.
 21. The high voltage power MOSFET ofclaim 15, wherein the first voltage regulating layer and the secondvoltage regulating layer are configured to accumulate charge carriers inan on state to reduce on-state resistance.